In terms of suppressing out-of-band distortion and canceling multi-path ISI, the standard design of most digital receivers cascades a MF with an adaptive equalizer. The MF usually takes the form of a square-root raised cosine (RRC) response in order to maximize SNR while the adaptive equalizer often operates as a fractional-spaced equalizer (FSE) to allow inverse modeling of the propagation channel across the full spectral band, as apposed to only at pass-band frequencies in the symbol-spaced equalizer.
In modern receivers the transmitter's pulse-shaping process re-proportions the spectral energy of the base-band signal such that the major spectral components occupy the low-frequency band while the minor spectral lobes are made to occupy the high frequency band. The band-limited representation of the transmitted signal infers that channel inversion at out-of-band frequencies is only as important to restoring full-received SNR as to the extent that the signal's high frequency spectral components are important to representing the distortion-less transmitted modulation.
In terms of suppressing such out-of-band distortion signals as CW jamming the FSE has historically relied upon both the recursion of its ISI-canceling operation and on the out-of-band attenuation characteristics of the MF preceding it. If the source of the distortion is thermal noise, however, the FSE must rely exclusively on the MF as the equalizer's spectral side-lobe levels are not well defined, are susceptible to variations in the adaptation constant, and therefore, cannot suppress the noise prior to decimation of the signal to the symbol rate.
With the importance of multi-path ISI cancellation relatively unimportant at out-of-band frequencies and with the pre-FSE RRC MF providing suppression of out-of-band interference, the responsibilities of the FSE's out-of-band mask have remained limited to the cancellation of excess adjacent channel interference not suppressed by the MF. It is noted that both the pre-FSE RRC MF and FSE are digital filters that operate at the same sample rate and whose responsibilities across the frequency band are approximately decoupled. Because of this, both filters can be combined into a single filter if control over the FSE's spectral mask can sustain well-defined side-lobes that are immune to changes in the equalizer's adaptation constant.
This suggests a single filter implementation for the cascade design. From the point of hardware, a cascading of successive digital filters demands that separate bank of FPGA multipliers must be used to service the demands of each filter in the cascade chain. A single filter implementation of the traditional RRC MF plus FSE cascade design reduces cost as similar hardware components can be used to service the processing of associated with each. Although an increase in computational complexity must result when two separate processes are combined to conserve hardware, many options present themselves to minimize this increase. Consequently, reductions in both computational complexity and power consumption over that required for the traditional cascade design are possible.
Accordingly, it is one objective of the present invention to provide a FSE that can simultaneously affect control over the equalizer's pass-band, roll-off, and side-lobe characteristics using a technique of constrained optimization so as to form a joint ISI-cancelling and MF update that can achieve the received SNR performance of the state-of-the-art cascade pre-FSE RRC MF plus FSE design.
It is another object of the present invention to provide a FSE that implements a time-multiplexing architecture that enables a single bank of multiplier elements to perform the inner product computations associated with both the ISI-canceling and MF updates within the confines of a constrained optimization update, the purpose to provide for reduced hardware complexity over that of the state-of-the-art cascade pre-FSE RRC MF plus FSE design.
It is the further object of the present invention to provide a FSE that operates as a joint ISI-canceling and MF FSE where the error associated with ISI-cancellation may be derived from any number of existing algorithms within the confines of the constrained optimization update of the present invention.
It is the further object of the present invention to interject the process of time-domain windowing of the constraint waveform into the constrained optimization update so as to minimize the increase in computational complexity incurred from the introduction of the time-multiplexing architecture.
It is the further object of the present invention to provide a FSE operating as a joint ISI-canceling and MF FSE where the rate at which the equalizer's weights are updated in accordance with the MF processing is controlled via an algorithm to minimize computational workload.
It is the further object of the present invention to provide a FSE operating as a joint ISI-canceling and MF adaptive equalizer which implements an initialization of the equalizer's FF weights using a selected set of coefficients of an RRC MF, the intent of which is to reduce the acquisition time of the MF characteristics of the FSE's steady-state joint inverse channel and MF function.
It is the further object of the present invention to provide a FSE operating as a joint and MF FSE under a constrained optimization update when the FSE is partitioned as a poly-phase process.